The present invention relates to a process and an apparatus for machining a workpiece by means of intermittent electro-erosive discharges applied between the electrode tool and the electrode workpiece of a wire electrode EDM machine. The process and apparatus of the invention permit to considerably improve the precision of the machining.
In wire electrode EDM machines, the electrode tool, in the form of a metallic electrically conductive wire continuously fed axially at a constant speed within the machining zone, is subjected to a displacement in a direction transverse to its longitudinal axis while following a predetermined cutting trajectory or path relative to the workpiece, at a relative speed predetermined as a function of the machining conditions. The trajectory or path, which is programmed as a function of the shape of the part to be obtained, takes into consideration the distance between the longitudinal axis of the wire electrode and the machined surfaces. It has been observed that the forward machining gap, between the leading edge of the wire electrode and the bottom of the cut in the workpiece, is less than the lateral gaps between the wire sides and the opposed wall surfaces of the cut in the workpiece. This is apparently due to the fact that some electro-erosion of the workpiece is effected laterally, simultaneously with the electro-erosion occurring at the leading edge of the wire electrode, as a result of the relatively long duration side machining which is effected between surfaces which are substantially parallel to the direction of feed of the wire electrode relative to the workpiece.
The distance separating the sidewall surfaces of the cut in the workpiece is a function of the diameter of the wire electrode and of the ratio of the speed of translation of the axis of the wire electrode to the feed speed of a tangent to the wire along the direction of advance or feed of the wire into the workpiece. When the wire electrode feed is along a rectilinear path, the ratio is equal to 1 and the axis of the wire is consequently positioned at the center of the distance separating the sidewall surfaces of the machined workpiece. However, when the path is along a curve, the relative speed of displacement of one of the lateral machining gaps increases with respect to the sidewall surfaces on the outside of the curve, and the relative speed of the other lateral machining gap decreases relative to the sidewall surface on the inside of the curve, such that the distance between the axis of the wire and the machined surfaces decreases with respect to the outer surface and increases with respect to the inner surface. There result some errors which affect the machining accuracy.
The error is even worse when the cutting path of the wire electrode is along an angular path. Under certain conditions, the lateral machining gap, at the vertex of the angular cut, is about of the same order of magnitude as the leading edge gap, and there results a machining error which may be greater than 10.mu..
The present invention permits to eliminate such a machining error. The present invention provides means for varying the speed of relative displacement between the wire electrode and the workpiece, as a function of the shape of the cutting path, for maintaining constant the distance between the axis of the wire electrode and one of the surfaces of the cut in the workpiece.
The present invention accomplishes its purpose by providing an adaptive control of the speed of feed of a wire electrode in an EDM machine which eliminated practically all the errors between the programmed cutting path of the wire electrode and the configuration of the machined workpiece in the portions of the cutting path effected along substantial curves.
A further advantage of the present invention is to obtain such results by means of a simple and inexpensive arrangement of elements, utilizing the information already present in the programmed cutting path of the wire electrode.